Fig. 1 Simplified diagram depicting the movement of carbon throughout the lithosphere (rock), hydrosphere (water), and atmosphere (air) as understood during SOIREE. Oceanic carbon inputs include particle deposition, biotic debris, atmospheric CO2 and photosynthesis/ respiration of phytoplankton, which then die and become organic carbon sediment to be incorporated into the lithosphere. Carbon that enters the lithosphere exits as inorganic calcium carbonate, which reacts with the low-pH ocean to produce CO2 to be released into the atmosphere, however, the majority remains sequestered on the ocean floor. It is hypothesized that the addition of iron, as it is a limiting nutrient, will accelerate the growth of photosynthetic phytoplankton, increasing the amount of sediment sequestering carbon into the ocean floor (Iron Hypothesis).